(1) Field of the Invention
The invention relates to a speed change gear control system for use in a hydrostatic-mechanical transmission which is equipped with a mechanical transmission unit actuated through an input shaft connectable to a power source; a hydrostatic transmission unit which is connectable to the input shaft and comprises a pump and motor having their respective discharge setting swash plates, the angle of at least either of the swash plates being variable; and a differential unit for actuating both the mechanical transmission unit and the hydrostatic transmission unit by connecting an output shaft thereto.
(2) Description of the Prior Art
One known clutch shift control method for hydrostatic-mechanical transmissions of the above type is disclosed in Japanese Patent Publication 1-30650 (1989). According to the clutch shift control method taught by this publication, relative revolution speed with respect to the disc plate is calculated for the clutch for either a higher speed range or lower speed range than the presently selected speed range. When the relative revolution speed thus calculated becomes a predetermined value or less, a gear shift instruction signal is generated to start the engagement of the higher speed clutch or lower speed clutch. Simultaneously with the engagement, the discharge volume of the pump in the hydrostatic-mechanical transmission is so controlled that the relative revolution speed of the higher or lower speed clutch comes into close proximity to zero. Then, it is determined from a signal sent from the higher or lower speed clutch whether the engagement of the clutch has been completed. If so, the disengagement of the clutch presently engaged starts, and thus the clutch shift control is completed.